Apertia · Library
Why is blur-circle diameter not a rating of bokeh quality?
50 mm at f/2 from 3 m and 100 mm at f/4 from 6 m both give a 0.4312 mm background disc and a 2087 mm frame, yet the photos can look different.
What the number captures
Apertia's bokeh figure is the diameter of the disc that a single point of the background spreads into, from the thin-lens formula b = f² ÷ (N(u − f)) for a background at infinity. The preview draws that disc to scale over a sample scene. It tells you how large out-of-focus points become; it says nothing about their shape, edge brightness, colour fringes or how a cluttered background renders.
Very different lenses can produce the same diameter. Once the diameter matches, the remaining differences come from the lens design and the scene, and the model has no input for either.
Compare two lenses fairly
- Fix the framing first: adjust the distance so both setups show the same scene width at the focus plane.
- Compare the blur diameters at that framing, with the background at the same distance.
- Take one test photo with each lens of the same background highlights.
- Describe the look you see, such as round or cat's-eye highlights, separately from the calculated size.
Worked example: equal size, different lenses
On full frame, a 50 mm lens at f/2 focused at 3000 mm and a 100 mm lens at f/4 focused at 6000 mm both frame a scene about 2087 mm wide. With the background at infinity, both give a disc of 0.4312 mm: f² ÷ (N(u − f)) is 2500 ÷ (2 × 2899.1) for the first and 10000 ÷ (4 × 5798.3) for the second. The model calls them identical. In practice the 100 mm shot also shows less of the background behind the subject, and the two lenses may draw highlights differently.
- 50 mm, f/2, focus 3000 mm
- 0.4312 mm · scene 2087 mm wide
- 100 mm, f/4, focus 6000 mm
- 0.4312 mm · scene 2087 mm wide
Common mistakes
- Calling the setup with the larger disc the one with better bokeh.
- Comparing lenses at the same distance instead of the same framing.
- Treating the preview's sample scene as a simulation of your lens's highlights.
- Expecting portrait-mode processing on a phone to follow the thin-lens disc.
Keep size and look apart
The diameter is a planning number for blur size only. The DOFMaster and University of Arizona references below cover the geometry; judging smoothness or character needs real photographs made under comparable conditions.